Nap-finished artificial leather such as suede-finished artificial leather and nubuck artificial leather which have a napped surface made of the fiber bundles on a substrate comprising fiber bundles and an elastic polymer has been known. The nap-finished artificial leather is required to fully satisfy a high level of physical properties such as fastness to light, pilling resistance and abrasion resistance, in addition to sensuous properties such as appearance (surface feeling closely resembling natural leather), hand (soft touch combined with moderate fullness and dense feeling), and color development (brilliantness and depth of color). To meet such requirements, there have been made various proposals.
To meet the requirement on the appearance and hand, for example, it has been generally employed to make artificial leather from microfine fibers. In the production of the artificial leather made of microfine fibers, it has been widely used to convert composite fibers such as sea-island fibers and multi-layered fibers to microfine fiber bundles by splitting or removal of a polymer component by decomposition or extraction. The nap-finished artificial leather and grain-finished artificial leather, which are made from a substrate for artificial leather comprising a nonwoven fabric of microfine fiber bundles derived from the composite fibers and an elastic polymer impregnated into the nonwoven fabric, are rated highly in their appearance and hand. However, such artificial leather involves a problem of lowering the color development as the fiber fineness is decreased, thereby to cause a remarkable deterioration in the brilliantness and depth of color. Particularly, the nap-finished artificial leather fails to meet a general requirement for high quality.
The nonwoven fabric for the substrate for artificial leather is generally produced by a method which includes a step of cutting spun fibers into staple fibers having a length of 100 mm or less, a step of making the staple fibers into a nonwoven web having a desired mass per unit area by a carding or paper making method, a step of optionally superposing two or more nonwoven webs, and a step of entangling the fibers by a needle-punching or spun-lacing method. Using the nonwoven fabric having a desired bulkiness and a degree of entanglement thus produced, the substrate for artificial leather is produced. The nap-finished artificial leather and grain-finished artificial leather produced from such a substrate for artificial leather are highly rated particularly in their hand. Although the staple fibers constituting the nonwoven fabric are fixed in the substrate by the entanglement between fibers and the impregnated elastic polymer, the staple fibers on the napped surface of nap-finished artificial leather or in the interface between the substrate and the grain layer of grain-finished artificial leather unavoidably tend to be easily pulled out or fallen from the nonwoven fabric because of their short length. With this tendency, the important surface properties such as the abrasion resistance of napped surface and the bonding/peeling strength of grain layer are reduced. To remove this problem, there have been generally employed to increase the degree of entanglement, bond the fibers with each other, or impregnate an elastic polymer in a large amount so as to strongly bind the fibers. However, the increase in the degree of entanglement and the use of an increased amount of elastic polymer in turn remarkably deteriorate the hand of artificial leather. Thus, it is difficult to satisfy the requirements for the appearance, hand and surface properties simultaneously.
To improve the surface abrasion resistance of nap-finished artificial leather, typically the pilling resistance of napped fibers, there has been proposed to produce suede-finished artificial leather by a method including a step of making a nonwoven fabric from sea-island fibers which are capable of being converted into bundles of microfine fibers of 0.8 D or less; a step of entangling the nonwoven fabric by needle punching; a step of immersing the entangled nonwoven fabric in an aqueous solution of polyvinyl alcohol (PVA) and then drying it to temporally fix the shape of the nonwoven fabric; a step of removing the sea component from the sea-island fibers by extraction using an organic solvent a step of impregnating a solution of polyurethane in dimethylformamide (DMF) and coagulating the polyurethane; and a step of raising the surface (Patent Document 1). It is also proposed to add coarse particles to the microfine fibers, the coarse particles having a particle size lager than a quarter of the fiber diameter and being inert to the fibers.
In Patent Document 2, it is proposed to produce suede-finished artificial leather by entangling a nonwoven fabric of sea-island fibers by needle punching; impregnating a solution of polyurethane in DMF into the entangled nonwoven fabric and coagulating the polyurethane; removing the sea component by extraction to obtain a leather-like substrate; and raising the obtained leather-like substrate. The fiber bundles constituting the substrate comprise fine fibers A of 0.02 to 0.2 D and microfine fibers B having a fineness of not more than ⅕ of the average fineness of the fine fibers A and less than 0.02 D. The ratio of the numbers of fibers (A/B) in fiber bundles is 2/1 to 2/3. The inside of fiber bundles is substantially free from an elastic polymer. The ratio of the number of fine fibers A and the number of the microfine fibers B (A/B) in the napped fibers is 3/1 or more.
There has been further proposed a method of improving the pilling resistance of suede-finished artificial leather, in which the foot of napped fibers is anchored by partially dissolving the elastic polymer around the foot of napped fibers using a solvent (Patent Document 3).
Patent Document 4 proposes a method of producing a nonwoven fabric of filaments which is capable of being converted into nubuck artificial leather having surface touch with fine texture. In the proposed method, the strain, which is characteristic of a nonwoven fabric of filaments and caused during the entangling treatment, is relieved by intentionally cutting the filaments during the entangling treatment by needle punching, thereby exposing the cut ends of fibers to the surface of nonwoven fabric in a density of 5 to 100/mm2. It is also proposed to regulate the number of fiber bundles within 5 to 70 per 1 cm width on the cross section parallel to the thickness direction of nonwoven fabric, i.e., regulate the number of fiber bundles which are oriented by needle punching toward the thickness direction within 5 to 70 per 1 cm width. It is further proposed to regulate the total area of fiber bundles on a cross section perpendicular to the thickness direction of nonwoven fabric within 5 to 70% of the cross-sectional area.
Patent Document 5 proposes an entangled nonwoven fabric made of filaments which are capable of being converted into microfine fibers of 0.5 D or less, in which the percentage crimp of filaments is 10% or less and the nonwoven fabric contains the fibers in a density of 0.25 to 0.50 g/cm3.
In the method of Patent Document 1, since the solution of polyurethane in DMF is impregnated and coagulated after removing the sea component of the sea-island fibers by extraction, the polyurethane penetrates into the inside of microfine fiber bundles, thereby making the hand hard. In addition, soft hand and touch are difficult to obtain because the coarse particles are added to the fibers.
In the method of Patent Document 2, since the solution of polyurethane in DMF is impregnated and coagulated before removing the sea component of sea-island fibers by extraction, the microfine fiber bundles are substantially free from the polyurethane on their outer surface and in their inside. Therefore, soft hand and touch are obtained. However, since the microfine fiber bundles are not fixed together by polyurethane, the pilling resistance is insufficient.
Patent Document 3 merely teaches to anchor the foot of napped fibers by partially dissolving the elastic polymer on the outermost surface of the leather-like substrate. Therefore, the fibers in the leather-like substrate are less fixed and the elastic polymer holds the fibers weakly. Therefore, the proposed method is not effective for improving the pilling resistance when the fineness is 0.01 dtex or more.
In the method of Patent Document 4 for obtaining the nonwoven fabric of filaments, the filaments are cut while preventing the properties from being made lower than intended. However, since a large number of filaments are actually cut, the advantages of filaments that the strength of nonwoven fabric is enhanced because of their continuity are reduced, thereby failing to effectively use their advantages. In Patent Document 4, the entangling treatment is not employed for entangling the filaments from the surface of nonwoven fabric of filaments, through the inside thereof, to the opposite surface, but employed for cutting the fibers on the surface of nonwoven fabric evenly to produce an extremely large number of cut ends as many as 5 to 100/mm2. Therefore, the entangling treatment should be performed by needle punching under conditions far severer than generally used. With needle punching under such severe conditions, the fibers are difficult to be well entangled and a number of filaments are changed to staple fibers to produce a nonwoven fabric which is different from a nonwoven fabric of entangled filaments. Artificial leather made from the resulting nonwoven fabric has grade and quality resembling those of known artificial leather made from staple fibers. Therefore, artificial leather having a high grade intended in the present invention is difficult to obtain in the proposed nonwoven fabric.
By the method of Patent Document 5, a relatively densified nonwoven fabric of filaments may be obtained when presumed from the degree of fabric density However, since the densification is made only by needle punching and press, gaps with a size of about a hundred to several hundreds of micrometer are present throughout the nonwoven fabric. With such a nonwoven fabric, it is difficult to obtain a high grade artificial leather intended in the present invention. In more detail, a needle-punched nonwoven fabric generally has gaps with a size of about several hundreds of micrometer to several millimeters, although depending upon the fiber diameter and needle-punching conditions. When such a nonwoven fabric is pressed in the thickness direction while softening a component of fibers under heating, the solidification of the sea component and the fixation of the shape are merely caused, and gaps remain although collapsed in the thickness direction. When the sea component is removed, the fixation by the sea component is lost and the collapsed gaps restore to the original size. Thus, the resulting nonwoven fabric has a structure in which gaps having a size of about a hundred to several hundreds of micrometer are scattered therein.    [Patent Document 1] JP 53-34903A (pages 3 and 4)    [Patent Document 2] JP 7-173778A (pages 1 and 2)    [Patent Document 3] JP 57-154468A (pages 1 and 2)    [Patent Document 4] JP 2000-273769A (pages 3 to 5)    [Patent Document 5] JP 11-200219A (pages 2 and 3)